diff --git a/extmod/extmod.cmake b/extmod/extmod.cmake
index 305cdee94276..7a7b892c9661 100644
--- a/extmod/extmod.cmake
+++ b/extmod/extmod.cmake
@@ -60,51 +60,62 @@ set(MICROPY_SOURCE_EXTMOD
 
 if(MICROPY_PY_BTREE)
     set(MICROPY_LIB_BERKELEY_DIR "${MICROPY_DIR}/lib/berkeley-db-1.xx")
+    string(CONCAT GIT_SUBMODULES "${GIT_SUBMODULES} " lib/berkeley-db-1.xx)
 
-    add_library(micropy_extmod_btree OBJECT
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_close.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_conv.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_debug.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_delete.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_get.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_open.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_overflow.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_page.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_put.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_search.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_seq.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_split.c
-        ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_utils.c
-        ${MICROPY_LIB_BERKELEY_DIR}/mpool/mpool.c
-    )
+    if(ECHO_SUBMODULES OR ECHO_QUERY_VARIANTS)
+        # No-op, we're just doing submodule/variant discovery.
+        # Cannot run the add_library/target_include_directories rules (even though
+        # the build won't run) because IDF will attempt verify the files exist.
+    elseif(NOT EXISTS ${MICROPY_LIB_BERKELEY_DIR}/README)
+        # Regular build, submodule not initialised -- fail with a clear error.
+        message(FATAL_ERROR " MICROPY_PY_BTREE is enabled but the berkeley-db submodule is not initialised.\n Run 'make BOARD=${MICROPY_BOARD} submodules'")
+    else()
+        # Regular build, we have the submodule.
+        add_library(micropy_extmod_btree OBJECT
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_close.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_conv.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_debug.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_delete.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_get.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_open.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_overflow.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_page.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_put.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_search.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_seq.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_split.c
+            ${MICROPY_LIB_BERKELEY_DIR}/btree/bt_utils.c
+            ${MICROPY_LIB_BERKELEY_DIR}/mpool/mpool.c
+        )
 
-    target_include_directories(micropy_extmod_btree PRIVATE
-        ${MICROPY_LIB_BERKELEY_DIR}/PORT/include
-    )
+        target_include_directories(micropy_extmod_btree PRIVATE
+            ${MICROPY_LIB_BERKELEY_DIR}/PORT/include
+        )
 
-    target_compile_definitions(micropy_extmod_btree PRIVATE
-        __DBINTERFACE_PRIVATE=1
-        mpool_error=printf
-        abort=abort_
-        "virt_fd_t=void*"
-    )
+        target_compile_definitions(micropy_extmod_btree PRIVATE
+            __DBINTERFACE_PRIVATE=1
+            mpool_error=printf
+            abort=abort_
+            "virt_fd_t=void*"
+        )
 
-    # The include directories and compile definitions below are needed to build
-    # modbtree.c and should be added to the main MicroPython target.
+        # The include directories and compile definitions below are needed to build
+        # modbtree.c and should be added to the main MicroPython target.
 
-    list(APPEND MICROPY_INC_CORE
-        "${MICROPY_LIB_BERKELEY_DIR}/PORT/include"
-    )
+        list(APPEND MICROPY_INC_CORE
+            "${MICROPY_LIB_BERKELEY_DIR}/PORT/include"
+        )
 
-    list(APPEND MICROPY_DEF_CORE
-        MICROPY_PY_BTREE=1
-        __DBINTERFACE_PRIVATE=1
-        "virt_fd_t=void*"
-    )
+        list(APPEND MICROPY_DEF_CORE
+            MICROPY_PY_BTREE=1
+            __DBINTERFACE_PRIVATE=1
+            "virt_fd_t=void*"
+        )
 
-    list(APPEND MICROPY_SOURCE_EXTMOD
-        ${MICROPY_EXTMOD_DIR}/modbtree.c
-    )
+        list(APPEND MICROPY_SOURCE_EXTMOD
+            ${MICROPY_EXTMOD_DIR}/modbtree.c
+        )
+    endif()
 endif()
 
 # Library for mbedtls
diff --git a/ports/esp32/Makefile b/ports/esp32/Makefile
index 7c8c225dfb6e..239618c6f73a 100644
--- a/ports/esp32/Makefile
+++ b/ports/esp32/Makefile
@@ -26,12 +26,6 @@ BAUD ?= 460800
 
 PYTHON ?= python3
 
-# Would be good to use cmake to discover submodules (see how rp2/Makefile does
-# it), but on ESP32 the same trick doesn't work because "idf.py build" fails
-# on berkeley-db dependency before printing out the submodule list.
-# For now just force the submodule dependencies here.
-GIT_SUBMODULES += lib/berkeley-db-1.xx lib/micropython-lib
-
 .PHONY: all clean deploy erase submodules FORCE
 
 CMAKE_ARGS =
@@ -40,20 +34,24 @@ ifdef USER_C_MODULES
 	CMAKE_ARGS += -DUSER_C_MODULES=${USER_C_MODULES}
 endif
 
-IDFPY_FLAGS += -D MICROPY_BOARD=$(BOARD) -D MICROPY_BOARD_DIR=$(abspath $(BOARD_DIR)) -B $(BUILD) $(CMAKE_ARGS)
+IDFPY_FLAGS += -D MICROPY_BOARD=$(BOARD) -D MICROPY_BOARD_DIR=$(abspath $(BOARD_DIR)) $(CMAKE_ARGS)
 
 ifdef FROZEN_MANIFEST
        IDFPY_FLAGS += -D MICROPY_FROZEN_MANIFEST=$(FROZEN_MANIFEST)
 endif
 
+ifdef BOARD_VARIANT
+	IDFPY_FLAGS += -D MICROPY_BOARD_VARIANT=$(BOARD_VARIANT)
+endif
+
 HELP_BUILD_ERROR ?= "See \033[1;31mhttps://github.com/micropython/micropython/wiki/Build-Troubleshooting\033[0m"
 
 define RUN_IDF_PY
-	idf.py $(IDFPY_FLAGS) -p $(PORT) -b $(BAUD) $(1)
+	idf.py $(IDFPY_FLAGS) -B $(BUILD) -p $(PORT) -b $(BAUD) $(1)
 endef
 
 all:
-	idf.py $(IDFPY_FLAGS) build || (echo -e $(HELP_BUILD_ERROR); false)
+	idf.py $(IDFPY_FLAGS) -B $(BUILD) build || (echo -e $(HELP_BUILD_ERROR); false)
 	@$(PYTHON) makeimg.py \
 		$(BUILD)/sdkconfig \
 		$(BUILD)/bootloader/bootloader.bin \
@@ -85,5 +83,12 @@ size-components:
 size-files:
 	$(call RUN_IDF_PY,size-files)
 
+# Running the build with ECHO_SUBMODULES set will trigger py/mkrules.cmake to
+# print out the value of the GIT_SUBMODULES variable, prefixed with
+# "GIT_SUBMODULES", and then abort. This extracts out that line from the idf.py
+# output and passes the list of submodules to py/mkrules.mk which does the
+# `git submodule init` on each.
 submodules:
-	$(MAKE) -f ../../py/mkrules.mk GIT_SUBMODULES="$(GIT_SUBMODULES)" submodules
+	@GIT_SUBMODULES=$$(idf.py $(IDFPY_FLAGS) -B $(BUILD)/submodules -D ECHO_SUBMODULES=1 build 2>&1 | \
+	                  grep '^GIT_SUBMODULES=' | cut -d= -f2); \
+	$(MAKE) -f ../../py/mkrules.mk GIT_SUBMODULES="$${GIT_SUBMODULES}" submodules
diff --git a/ports/esp32/esp32_common.cmake b/ports/esp32/esp32_common.cmake
index 37a19316b603..098df1f1ae85 100644
--- a/ports/esp32/esp32_common.cmake
+++ b/ports/esp32/esp32_common.cmake
@@ -11,6 +11,9 @@ endif()
 # Include core source components.
 include(${MICROPY_DIR}/py/py.cmake)
 
+# CMAKE_BUILD_EARLY_EXPANSION is set during the component-discovery phase of
+# `idf.py build`, so none of the extmod/usermod (and in reality, most of the
+# micropython) rules need to happen. Specifically, you cannot invoke add_library.
 if(NOT CMAKE_BUILD_EARLY_EXPANSION)
     # Enable extmod components that will be configured by extmod.cmake.
     # A board may also have enabled additional components.
diff --git a/ports/rp2/Makefile b/ports/rp2/Makefile
index 4f5b4728ecbc..7c14415ad556 100644
--- a/ports/rp2/Makefile
+++ b/ports/rp2/Makefile
@@ -50,9 +50,14 @@ clean:
 
 # First ensure that pico-sdk is initialised, then use cmake to pick everything
 # else (including board-specific dependencies).
+# Running the build with ECHO_SUBMODULES set will trigger py/mkrules.cmake to
+# print out the value of the GIT_SUBMODULES variable, prefixed with
+# "GIT_SUBMODULES", and then abort. This extracts out that line from the cmake
+# output and passes the list of submodules to py/mkrules.mk which does the
+# `git submodule init` on each.
 submodules:
 	$(MAKE) -f ../../py/mkrules.mk GIT_SUBMODULES="lib/pico-sdk" submodules
-	GIT_SUBMODULES=$$(cmake -B $(BUILD)/submodules -DECHO_SUBMODULES=1 ${CMAKE_ARGS} -S . 2>&1 | \
+	@GIT_SUBMODULES=$$(cmake -B $(BUILD)/submodules -DECHO_SUBMODULES=1 ${CMAKE_ARGS} -S . 2>&1 | \
 	                  grep '^GIT_SUBMODULES=' | cut -d= -f2); \
 	$(MAKE) -f ../../py/mkrules.mk GIT_SUBMODULES="$${GIT_SUBMODULES}" submodules
 
diff --git a/py/mkrules.cmake b/py/mkrules.cmake
index 7eb5fcf01837..21d5c27334e3 100644
--- a/py/mkrules.cmake
+++ b/py/mkrules.cmake
@@ -178,7 +178,12 @@ if(MICROPY_FROZEN_MANIFEST)
         set(MICROPY_LIB_DIR ${MICROPY_DIR}/lib/micropython-lib)
     endif()
 
-    if(NOT (${ECHO_SUBMODULES}) AND NOT EXISTS ${MICROPY_LIB_DIR}/README.md)
+    if(ECHO_SUBMODULES OR ECHO_QUERY_VARIANTS)
+        # No-op, we're just doing submodule/variant discovery.
+        # Note: All the following rules are safe to run in discovery mode even
+        # though the submodule might not be available as they do not directly depend
+        # on anything from the submodule.
+    elseif(NOT EXISTS ${MICROPY_LIB_DIR}/README.md)
         message(FATAL_ERROR " micropython-lib not initialized.\n Run 'make BOARD=${MICROPY_BOARD} submodules'")
     endif()